This application claims priority from British Application Number 0122153.0, filed Sep. 13, 2001.
This invention relates to a catalyzed urea formaldehyde binder for use in abrasive articles, a method of making the binder, abrasive articles made therewith and in particular to coated abrasive articles and to a method of making coated abrasive articles.
Coated abrasive articles generally contain an abrasive material, typically in the form of abrasive grains, bonded to a backing via of one or more adhesive layers. Such articles usually take the form of sheets, discs, belts, bands, and the like, which can be adapted to be mounted on pads, wheels or drums. Abrasive articles can be used for sanding, grinding or polishing various surfaces of, for example, steel and other metals, wood, wood-like laminates, plastic, fiberglass, leather or ceramics.
The backings used in coated abrasive articles are typically made of paper, polymeric materials, cloth, vulcanized fiber or combinations of these materials. A common type of bond system includes a make coat, a size coat, and optionally a supersize coat. The make coat typically includes a tough, resilient polymer binder that adheres the abrasive particles to the backing. The size coat, which also typically includes a tough resilient polymer binder that may be the same as or different from the make coat binder, is applied over the make coat and abrasive particles to further reinforce the particles. The supersize coat, including one or more antiloading ingredients or perhaps grinding aids, may then be applied over the size coat if desired.
In a typical manufacturing process, a coated abrasive article is made in a continuous web form and then converted into a desired construction, such as a sheet, disc, belt, or the like. Binders for the purpose of adhering the abrasive granules to the backing include the traditional phenolic resins, urea-formaldehyde resins, hide glue, varnish, epoxy resins, and polyurethane resins, or more recently a class of radiation cured crosslinked acrylate binders; see, e.g., in U.S. Pat. No. 4,751,138 (Tumey, et al.) and U.S. Pat. No. 4,828,583 (Oxman, et al.).
High performance coated abrasive articles have traditionally used phenolic size resins. Such resin systems suffer from the disadvantage that they require high temperatures for a prolonged time for optimum curing. This prevents the use of such resins with some polymeric backings either because they will not withstand the cure temperature or because the high cure temperature may result in dimensional instability of the coated sheet, e.g., curling upon cooling to ambient temperature. Additional disadvantages are that phenolic resins tend to be more expensive and have more undesirable emissions compared to urea-formaldehyde resin systems.
Urea formaldehyde (UF) was first patented for use as an adhesive for coated abrasives by 3M Company (xe2x80x9c3Mxe2x80x9d) in the mid 1930""s (Great Britain Pat. No. 419,812). Since that time a number of different coated abrasive products have been made with acid catalyzed UF resins. Today, the two most common catalysts used with UF resins are aluminum chloride (AlCl3) and ammonium chloride (NH4Cl).
Urea-aldehyde resins have enjoyed great success in coated abrasives. However, the need to reduce the use of solvents and unreacted reactants which contribute to release of volatile organic hydrocarbons (VOC) in the process of making coated abrasives and the need to increase the quality of the abrasives while maintaining or increasing their level of performance are challenging the industry.
When aluminum chloride is used as the catalyst, a higher temperature than normal must be used to cure the urea-aldehyde resin, which in turn leads to curling of edges of the coated abrasive. Also, the gel time, pot life and peak exotherm temperatures are all dependent on the concentration of the aluminum chloride. Consequently, there is a trade-off between aluminum chloride concentration and curing conditions, especially with low free-aldehyde UF resins.
Unlike aluminum chloride catalysis, the gel time, pot life and peak exotherm temperatures are all independent of the ammonium chloride concentration. However, the activity (ability of the catalyst to catalyze the reaction) of ammonium chloride is dependent on the free formaldehyde concentration in the binder precursor composition. With low free aldehyde resins, the ammonium chloride does not activate the condensation reaction very readily until a sufficient temperature is reached. However, as mentioned above, increased temperature tends to curl the edges of the coated abrasive and does not render performance improvements.
U.S. Pat. No. 5,611,825 (Engen, et al.) reports coated abrasives comprising a backing coated on at least one major surface thereof with an abrasive coating comprising a binder and abrasive particles. The binder is comprised of a solidified urea-aldehyde resin, the solidified urea-aldehyde resin being derived from a binder precursor comprising a urea-aldehyde resin having a low free aldehyde content and a co-catalyst. The co-catalyst is a catalyst consisting essentially of a Lewis acid, preferably aluminium chloride or an organic amine salt or an ammonium salt, preferably ammonium chloride. Preferred linear organic amine salts are those selected from the group of compounds having the general formula:
(X31 )+H3N(CH2)nNH3+(Yxe2x88x92)
wherein X and Y are halide atoms that may be the same or different and n is an integer ranging from about 3 to about 10. An example of such a linear organic amine salt found useful is the dichloride salt of hexamethylene diamine, obtained by the acidification of an aqueous solution of hexamethylene diamine with hydrochloric acid (HCl). One branched chain organic amine salt found useful is that known under the trade designation xe2x80x9cDYTEK-A,xe2x80x9d available from E. I. duPont de Nemours and Co., Wilmington, Del., which is commonly known as 2-methyl-pentamethylene diamine.
Although urea-formaldehyde resins have been used as make, size and supersize resins in coated abrasives they are generally not able to match the performance of coated abrasive made with phenol-formaldehyde resins.
It has now been found that certain urea formaldehyde resin systems can provide comparable performance to phenol formaldehyde resins when used in the production of coated abrasives. According to the present invention there is provided a coated abrasive article comprising a backing having at least one major surface, a plurality of abrasive grains bonded to at least a portion of the one major surface of the backing by at least one binder, wherein the binder comprises an urea formaldehyde resin precursor cured in the presence of a sole catalyst which consists essentially of at least one salt of an acid with a diamine of the formula:
H2Nxe2x80x94Rxe2x80x94NH2
in which R is an alkylene group of 3 to 10 carbon atoms, wherein the acid is selected from the group consisting of hydrochloric, citric, nitric, sulphuric, acetic, phosphoric and combinations thereof.
In a further aspect, the invention provides a method of making a coated abrasive which comprises coating a major surface of a backing with a plurality of abrasive grains and a binder comprising a urea formaldehyde resin precursor solution and a solution of a sole catalyst which consists essentially of at least one salt of an acid with a diamine of the formula:
H2Nxe2x80x94Rxe2x80x94NH2
in which R is an alkylene group of 3 to 10 carbon atoms, wherein the acid is selected from the group consisting of hydrochloric, citric, nitric, sulphuric, acetic, phosphoric and combinations thereof, and curing the urea formaldehyde resin precursor. Curing is typically accomplished by heating at a temperature of at least 60xc2x0 C., preferably at a temperature in the range of about 75xc2x0 C. to 140xc2x0 C., or a temperature in the range of 80xc2x0 C. to 90xc2x0 C. for 40 minutes or less, or at a temperature in the range of 115xc2x0 C. to 125xc2x0 C. for less than 10 minutes.
In a further aspect, the invention provides a binder which is useful in abrasive products comprising urea formaldehyde precursor resin cured in the presence of a sole catalyst which consists essentially of at least one salt of an acid with a diamine of the formula:
H2Nxe2x80x94Rxe2x80x94NH2
in which R is an alkylene group of 3 to 10 carbon atoms, wherein the acid is selected from the group consisting of hydrochloric, citric, nitric, sulphuric, acetic, phosphoric and combinations thereof.
In a further aspect, the invention provides a method of making a binder comprising mixing components comprising an aqueous solution of a urea-formaldehyde resin precursor; and a sufficient quantity of an aqueous solution of a sole catalyst to initiate cross-linking of said urea-formaldehyde resin precursor of a catalyst consisting essentially of at least one salt of an acid with a diamine of the formula:
H2Nxe2x80x94Rxe2x80x94NH2
in which R is an alkylene group of 3 to 10 carbon atoms, wherein the acid is selected from the group consisting of hydrochloric, citric, nitric, sulphuric, acetic, phosphoric and combinations thereof to provide a mixture; and heating said mixture to provide said binder.
As used herein, the term xe2x80x9csole catalystxe2x80x9d means only one catalyst is employed, that being the diamine salt catalyst as defined above.
The term xe2x80x9ccatalystxe2x80x9d refers to the diamine salt defined above and its ability to initiate polymerization of urea-formaldehyde resin precursor to provide a cured urea-formaldehyde resin which is cross-linked.
It has been found that the use of particular catalysts which are salts of a lower alkaline diamine with an acid in combination with urea formaldehyde resin precursors provide a binder system suitable for use in coated abrasives which may provide comparable and sometimes superior physical properties to the use of phenolic resins systems while allowing low cure temperatures and shorter cure times. The cost of the urea formaldehyde binder system is significantly less than the cost of a phenolic resin system and the urea formaldehyde resin may have in excess of 90% less emissions than a phenolic resin system.
The catalyst used in the invention is derived from an alkaline diamine containing from 3 to 10 carbon atoms. Preferably, the diamine is 1,2 hexamethylene diamine or octadiamine. The acid is selected from hydrochloric, citric, nitric, sulphuric, acetic and phosphoric acids. Phosphoric acid is preferred. The preferred catalyst is 1,6, hexamethylene diamine phosphate.
The catalysts that are useful to initiate the cure of urea formaldehyde resin precursor in accordance with the present invention are formed by reacting the diamine with an acid to form a salt. The diamines are typically reacted to give a salt solution with a pH in the range of about 10.0 to about 10.5 for the HCl salt and about 6 for the phosphate and other acid salts. The optimum pH depends upon the acid used and is generally about 6, except for the chloride salt. The urea formaldehyde resin precursor, typically available as an aqueous solution, is mixed with an aqueous solution of the diamine salt catalyst and heated to cause the resin precursor to cure.
The catalysts are typically used in an amount just sufficient to initiate the reaction to cause urea formaldehyde precursor to polymerize to form the urea formaldehyde resin, although additional amounts may also be useful. That amount of diamine catalyst on a dry weight basis is typically in the range of about 1 to about 25% by weight, preferably about 2 to about 10% by weight, most preferably about 3 to about 5% by weight, based upon the total dry weight of the urea formaldehyde resin precursor plus diamine catalyst.
It has been surprisingly found that these diamine salt catalysts in combination with urea formaldehyde resin precursors provide improved cured urea formaldehyde resin binders compared with those produced by the use of the corresponding triamines or hexamine catalyst systems.
The above defined diamine salts are the sole catalysts employed in the urea formaldehyde binders of the invention. The diamine salts are latent catalysts and do not catalyze the curing of the resin below temperatures of about 60xc2x0 C. Thus, the pot life of the resin binder system at ambient temperature is longer which is particularly beneficial in the manufacturing process of the coated abrasives. This is in contrast to a co-catalyst system comprising a Lewis acid, such as aluminium chloride, and an amine salt which begins to cure the resin system at ambient temperature and has a limited pot life.
The term xe2x80x9curea formaldehyde resin precursorxe2x80x9d refers to compounds which may include monomers or oligomers which are curable in the presence of an appropriate catalyst to provide fully cured urea formaldehyde resins which are solid polymeric materials that are cross-linked. Urea-formaldehyde resin precursors compositions useful in the present invention may be prepared by the reaction of urea with formaldehyde. The molar ratio of formaldehyde to urea (xe2x80x9cF/U ratioxe2x80x9d) of the resin ranges from about 1.4:1.0 to about 1.6:1.0. Urea-formaldehyde resins having low, i.e. less than 1%, free formaldehyde are preferred. The urea formaldehyde resin precursor aqueous solution generally has a viscosity in the range 600 to 1600 cps (0.6 to 1.6 Pascal seconds) measured at 60% by weight solids in aqueous medium using a BROOKFIELD LV viscometer with a number 1 spindle at ambient temperature (e.g., 20xc2x0 C.). A preferred urea formaldehyde resin has a viscosity of about 860 cps (0.86 Pascal seconds) at ambient temperature.
Examples of commercially available urea-formaldehyde resin precursor aqueous solutions include those having the trade designations xe2x80x9cAL3029R,xe2x80x9d commercially available from the Borden Chemical Co., Westchester, Ill., USA, and xe2x80x9cCBU UF,xe2x80x9d commercially available from Dynochem Limited, Mold, U.K.
The binder preferably generally additionally comprises at least one of an acid filler or neutral filler. Preferred fillers are of the platelet type having a particle size of less than 10 micrometers. Preferred fillers include mica and clays (e.g., kaolin and silane-treated kaolin). Calcium silicate, magnesium calcium silicate may also be used. Specific materials suitable for use as fillers include those under the trade designations: SX400 mica, VANSIL EW20 (Wollastonite, calcium silicate), NYTAL 200, 400 and 7700 (magnesium calcium silicate, Microfine Minerals Ltd., Derby, U.K.); POLARITE 102A (silane treated calcined kaolin), POLESTAR 200R (calcined kaolin), kaolin grade E-silane treated, Supreme China Clay (Imerys Co., Paris, France).
The filler is generally employed in an amount from about 5 to about 50% by weight of the dry weight of urea formaldehyde binder (that being the dry weight of the urea formaldehyde precursor plus the dry weight of the diamine catalyst), preferably from about 15 to about 30%, more preferably about 25% by weight of the dry weight of the urea formaldehyde binder. The presence of the filler contributes towards the flexural modulus of the cured binder system.
The binder preferably comprises a wetting agent to assist in defloculating and dispersing the filler. The particular selection of wetting agent will depend upon the filler present in the binder formulation. Suitable wetting agents include esters of polyethylene glycol, ammonium salt of polyacrylic acid and a methacrylamide functional amine adduct of neopentyl-diallyl-oxy-tridioctyl pyro-phosphato titanate.
Suitable materials for use as wetting agents for the fillers include those available under the trade designations: DISPEX A40 (ammonium salt of polyacrylic acid, Harcros Chemicals, Inc., Kansas City, Kans.), IRGASTAT 33 (ester of polyethylene glycol, Ciba Specialty Chemicals, Basel, Switzerland), LICA 38J (methacrylamide functional amine adduct of neopentyl-diallyl-oxy tri-dioctyl pyro-phosphato titanate, Kenrich Petrochemicals Inc., Bayonne, N.J.).
The wetting agent is generally used in the range about 0.1 to about 1.0% by weight based on the total weight of filler, although additional amounts may also be useful.
The binder formulations used in the invention may preferably additionally comprise a toughening agent. This is preferably a polymer latex selected from vinyl acetate, vinyl chloride, ethylene, styrene butyl acrylate and vinyl ester of versatic acid, polymers and copolymers.
The glass transition temperature (Tg) of the polymers used as toughening agents is typically in the range 0xc2x0 C. to 50xc2x0 C. Typical useful polymers include VINAMUL, e.g., VINAMUL 3303 (vinyl acetate-ethylene, Tg 0xc2x0 C.), VINAMUL 3405 (a blend of the monomers vinyl acetate, vinyl chloride and ethylene with nonylphenol ethoxylate surfactant as a dispersant), VINAMUL 3479 (vinyl acetate-vinyl chloride-ethylene, Tg 30xc2x0 C.), VINAMUL 69223 (vinyl acetate-vinyl ester of versatic acid, Tg 22xc2x0 C.), VINAMUL 3252 (vinyl acetate-ethylene, Tg 3xc2x0 C.), VINAMUL 3253 (vinyl acetate-ethylene, Tg 7xc2x0 C.), VINAMUL 31259 (vinyl acetate-ethylene), VINAMUL 3171 (vinyl acetate-ethylene, Tg 4xc2x0 C.), VINAMUL 43627 (vinyl acetate-butyl acrylate), and VINAMUL 7139 (Styrene-acrylate, Tg 50xc2x0 C.), commercially available from Vinamul Polymers, Bridgewater, N.J.
The toughening agent is generally present in an amount in the range about 1 to about 50% by weight based on the weight of the urea formaldehyde resin (i.e., the resin precursor plus catalyst).
The binder formulation may additionally comprise other adjuvants, e.g., a defoamer and other conventional adjuvants typically used in coated abrasive binder formulations.
The urea formaldehyde binder may be present as a make coat, size coat and/or a supersize coat. Preferably, the binder is used as a size coat. The binder may be coated by any of the conventional techniques known in the art. The binder is generally cured at a temperature in the range of 75 to 140xc2x0 C. Low temperature curing can be effected at a temperature of 80 to 90xc2x0 C. for 20 to 40 minutes. Alternatively, higher temperatures may be employed (e.g., 115 to 125xc2x0 C.) for shorter cure time periods (e.g., less than 10 minutes). Resin slabs are typically pre-dried at lower temperatures (e.g., 50xc2x0 C.) prior to curing.
When used as a supersize coat, the binder formulation may comprise antiloading agents, fillers, anti-static agents, lubricants, grinding aids, etc. Examples of such additives include salts and soaps of fatty acids such as lauric acid, myristic acid, palmitic acid, stearic acid, arachic acid and behenic acid, stearate salts, particularly calcium, zinc and lithium stearate, fluorinated compounds, e.g., a fluorochemical compound selected from compounds comprising a fluorinated aliphatic group attached to a polar group or moiety and compounds having a molecular weight of at least about 750 and comprising a non-fluorinated polymeric backbone having a plurality of pendant fluorinated aliphatic groups comprising the higher of (a) a minimum of three Cxe2x80x94F bonds, or (b) in which 25% of the Cxe2x80x94H bonds have been replaced by Cxe2x80x94F bonds such that the fluorochemical compounds comprises at least 15% by weight of fluorine, potassium fluoroborate, sodium fluorosilicate, potassium fluoride, iron sulfide, potassium phosphate, molybdenum disulfide and calcium hydrogen phosphate and the anti-loading component disclosed in U.S. Pat. No. 5,704,952 (Law, et al.) incorporated herein by reference.
The backing substrate used in the coated abrasive articles may be selected from any of a wide range of materials including paper, polymeric materials, cloth, and combinations thereof.
The abrasive articles can contain 100% of a single abrasive grain mineral composition. Alternatively, the abrasive article may comprise a blend or mixture of different abrasive grain mineral compositions. The mineral may be coated from 1% to 99% blends, preferably 50 to 95%, to form either open or closed coat construction. Useful conventional abrasive grains include fused aluminum oxide, heat treated aluminum oxide, white fused aluminum oxide, silica, silicon carbide, titanium diboride, boron carbide, tungsten carbide, titanium carbide, diamond, cubic boron nitride, garnet, fused alumina zirconia, sol gel abrasive grains and the like. Examples of sol gel abrasive grains can be found in U.S. Pat. No. 4,314,827 (Leitheiser, et al.); U.S. Pat. No. 4,623,364 (Cottringer, et al.); U.S. Pat. No. 4,744,802 (Schwabel); U.S. Pat. No. 4,770,671 (Monroe, et al.) and U.S. Pat. No. 4,881,951 (Wood, et al.), all of which are incorporated herein by reference. The diamond and cubic boron nitride abrasive grains may be monocrystalline or polycrystalline. The particle size of these conventional abrasive grains can range from about 0.01 to 1500 micrometers, typically between 1 to 1000 micrometers. The abrasive grains may also contain an organic or inorganic coating. Such surface coatings are described, for example, in U.S. Pat. No. 5,011,508 (Wald, et al.); U.S. Pat. No. 1,910,444 (Nicholson); U.S. Pat. No. 3,041,156 (Rowse, et al.); U.S. Pat. No. 5,009,675 (Kunz, et al.); U.S. Pat. No. 4,997,461 (Markhoff-Metheny); U.S. Pat. No. 5,213,591 (Celikkaya, et al.); U.S. Pat. No. 5,085,671 (Martin, et al.); and U.S. Pat. No. 5,042,991 (Kunz, et al.) all of which are incorporated herein by reference.
In one embodiment a pressure sensitive adhesive is coated onto the back side of the coated abrasive such that the resulting coated abrasive can be secured to a back up pad. In another embodiment the coated abrasive may contain a hook and loop type attachment system to secure the coated abrasive to the back up pad. The loop fabric may be on the back side of the coated abrasive with hooks on the back up pad. Alternatively, the hooks may be on the back side of the coated abrasive with the loops on the back up pad. This hook and loop type attachment system is further described in U.S. Pat. No. 4,609,581 (Ott); U.S. Pat. No. 5,254,194 (Ott, et al.); and U.S. Pat. No. 5,505,747 (Barry, et al.), all of which are incorporated herein by reference.